1. Technical Field
The present invention relates to an end-face seal for graphite electrodes, especially for large diameter graphite electrodes, a process for preparing the inventive end-face seal. More particularly, the invention concerns a multi-part seal used at the end faces of graphite electrodes to protect the surfaces of an electrode joint from oxidation. The seal can be efficiently produced and transported and is preferably formed of particles of expanded graphite
2. Background Art
Graphite electrodes are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Electrical currents in excess of 100,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Generally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.
Alternatively, the electrodes can be formed with a male threaded protrusion or stub machined into one end and a female threaded socket machined into the other end, such that the electrodes can be joined by threading the male stub of one electrode into the female socket of a second electrode, and thus form an electrode column. The joined ends of two adjoining electrodes in such an embodiment is also referred to in the art as a joint.
Given the extreme thermal stress that the electrode and the joint (and indeed the electrode column as a whole) undergoes, mechanical/thermal factors such as strength, thermal expansion, and crack resistance must be carefully balanced to avoid damage or destruction of the electrode column or individual electrodes. For instance, longitudinal (i.e., along the length of the electrode/electrode column) thermal expansion of the electrodes, especially at a rate different than that of the pin, can force the joint apart, reducing effectiveness of the electrode column in conducting the electrical current. A certain amount of transverse (i.e., across the diameter of the electrode/electrode column) thermal expansion of the electrode in excess of that of the pin may be desirable to form a firm connection between pin and electrode; however, if the transverse thermal expansion of the electrode greatly exceeds that of the pin, damage to the electrode or separation of the joint may result. Again, this can result in reduced effectiveness of the electrode column, or even destruction of the column if the damage is so severe that the electrode column fails at the joint section.
Moreover, another effect of the thermal and mechanical stresses to which an electrode column is exposed is a gap forming in the electrode joint between two adjoining electrodes. Moreover, the engineering of the pin and female sockets, or male stub and female socket, can lead to a gap intentionally existing between the two adjoining electrodes in a joint. When a gap forms in a joint, either intentionally or unintentionally, oxidation of the threads of the pin or male stub and other joint surfaces can occur, resulting in loss of material and what is referred to as “necking.” Necking occurs when sufficient material is oxidized away from the joint surfaces to narrow and thus weaken the joint between the two end faces. Necking reduces effectiveness of the electrode column in conducting the electrical current, reduces the mechanical strength of the joint, and can ultimately lead to failure of the joint and catastrophic loss of the electrode column. For instance, when necking occurs to a significant degree, the vibrations normally experienced by an electrode column in use in the furnace can lead to cracks and ruptures in the pin or male stub, or the female electrode engaged by the pin or socket, and separation of the joint and loss of the electrode column below the affected joint.
In U.S. Pat. No. 3,540,764, Paus and Revilock suggest the use of an expanded graphite spacer disposed between the end faces of adjacent electrodes in order to increase electrical conductivity and thermal stress resistance of the joint. The nature of the Paus and Revilock spacer and its placement, however, is such that a gap is created in the joint where it may not have otherwise been, thereby contributing to joint looseness and potential for failure.
Moreover, because of the diameter of graphite electrodes, which can be 750 millimeters (mm) or greater in diameter, materials intended for use between adjoining electrodes are approximately the same diameter, with a central opening at least as large in diameter as the pin or male stub used to join adjoining electrodes. Because of this, transport of such materials can be problematic, both because of the size of the materials being transported and because the structural integrity of large cylindrical materials may not be able to withstand the stress of transport. In addition, depending on the productions methods employed, the production of large diameter materials with large central openings can result in a great deal of waste material.
What is desired, therefore, is a seal for reducing oxidation at the surfaces portions of graphite electrode joints, which can be formed as a plurality of individual elements for transport. It is also highly desirable to achieve these property benefits without using high quantities of expensive materials.